The present invention relates to a piston for an internal combustion engine.
As is generally known, due to the action of inertia force, thrust or side force resulting from combustion pressure upon expansion stroke, and moment about a piston pin, a piston for an internal combustion engine for use, e.g., in a motor vehicle is inclined during vertical reciprocating motion of one cycle, and has thrust side and counter thrust side portions of a skirt pressed on an inner surface of a cylinder.
By way of example, referring to FIG. 7, in the vicinity of the upper dead center of expansion stroke, combustion pressure makes a piston 1 produce clockwise moment M and thrust force TF on a piston pin 3 through a con'rod 2. As a result, the piston 1 is inclined so that an upper end of a counter thrust side skirt 6b or crown 4 comes in contact C with a counter thrust side inner surface 5a of a cylinder 5, whereas a lower portion of a thrust side skirt 6a comes in contact with a thrust side inner surface 5b of the cylinder 5. Subsequently, as the piston 1 goes down, gas pressure and con'rod angle are increased to augment thrust. As a result, the piston 1 is pressed on the thrust side inner surface 5b of the cylinder 5, and the crown 4 is moved from the thrust side TS to the counter thrust side CTS. The piston 1 separates from the cylinder 5 on the counter thrust side, and has a contact center position on the thrust side which moves from the lower portion of the thrust side skirt 6a to an upper portion thereof. At that moment, the thrust side skirt 6a undergoes maximum thrust during one cycle, so that the entirety of a skirt 6 is deformed along the cylinder inner surface 5b, and has increased contact area with the cylinder inner surface 5b, resulting in great friction of the skirt 6.
In view of such circumstances of the art, JP-A 179154 proposes a piston having partly thick portions arranged on the reverse side of the skirt in the thrust and counter thrust directions. A step-like rigidity change is produced between each thick portion and a thin portion arranged adjacent thereto in the circumferential direction. Thus, when undergoing great side force from the cylinder inner surface, the corresponding thick portion is pressed inside without any deformation, so that the adjacent thin portion is deformed contrariwise to expand outside and come in contact with the cylinder inner surface, decreasing a contact area, resulting in reduced friction of the skirt.
With the conventional piston disclosed in JP-A 179154, however, the partly thick portions are arranged on the reverse side of the lower portion of the skirt, so that when great clockwise moment is produced in the vicinity of the upper dead center of expansion stroke as described above, the lower portion of the skirt undergoes great deformation due to a structure that the thrust side skirt has a thin portion between the thick portion and a pin boss, resulting in increased incline amount of the piston. Therefore, immediately after the upper dead center, the counter thrust side crown may crash violently against the cylinder inner surface to produce hammering.
Further, in a low-speed and low-load state after engine start, when going down after the upper dead center, the piston has a thrust side contact center position which moves from the lower portion of the skirt to the upper portion thereof. At that moment, due to smooth movement of the contact center position, cold slap can be restricted. However, since the partly thick portions are arranged on the reverse side of the lower portion of the skirt, the piston starts to contact the cylinder inner surface from a lower end of the thrust side skirt. The contact center position moves quickly to the thick portion of the lower portion of the skirt, and stays temporarily therein since thrust is supported by the thick portion. As soon as a load of thrust cannot be supported by the thick portion due to increased thrust, the contact center position moves quickly from the lower portion of the skirt to the center of the piston pin. Thus, cold slap may be produced.
Furthermore, due to increased incline amount of the piston, the piston crown should have an outer diameter determined at a relatively small value. This results in enlarged land clearance between an outer periphery of the piston crown and the cylinder inner surface. Thus, a crevice volume is increased in the vicinity of a top land, increasing a production of unburnt hydro-carbon (HC), resulting in lowering of exhaust emission performance.
It is, therefore, an object of the present invention to provide a piston for an internal combustion engine which enables a restraint of occurrence of hammering and cold slap and a reduction in frictional resistance.